3 Answers

I believe Samsung, Apple, and the other major manufactures make CAD data of the device bodies available on their websites.
If you are designing something (i.e. a cover), don't trust a model from GrabCAD (or another site). Obtain it from the device manufacturer if you value your time and money.
If it is not being use to design something, then sure, use any model that looks correct.
You can also try searching these sites:
https://www.cgtrader.com/www.turbosquid.com

It has been some time since I worked on a phone project, but Apple had a (hard to get to) portion of their website setup for designers.
From the site you'd be able to download a "dumb solid" of the desired phone. I have to think that Samsung and other manufacturers have the same type of setup.
Hopefully someone on this site will know exactly how to get to the correct site.

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